1. Field of the Invention
The present invention relates to a zoom lens system, and in particular, relates to a large-aperture telephoto zoom lens system that is favorable for use in an imaging apparatus such as a digital camera, etc.
2. Description of Related Art
In recent years, optical systems used in imaging apparatuses such as digital cameras, etc., high-speed focusing has been in demand in addition to high optical quality, and hence, various optical systems have been proposed in which the focusing lens group provided therein has been miniaturized and reduced in weight. However, in optical systems of the related art, the balance between achieving a favorable optical quality and a favorable miniaturization/reduction in weight of the focusing lens group is at an unsatisfactory level.
In Patent Literature Nos. 1 through 4, in the case where the lens groups are defined (divided from each other) at each air-space between adjacent lens elements that varies in distance therebetween during zooming, a configuration of four lens groups (Patent Literature Nos. 1, 2 and 4) and a configuration of five lens group (Patent Literature No. 3) are disclosed. However, since the balance in refractive power between the lens groups in each zoom lens system in Patent Literature Nos. 1 through 4 is inappropriate, correction of various aberrations such as spherical aberration, coma and distortion, etc., is insufficient. Furthermore, in the zoom lens systems of Patent Literature Nos. 1 and 2, focusing is carried out using a rear half of the first lens group, which has the largest diameter and is heavy, so that the driving load for performing an autofocusing operation is large, and a large change in optical quality (deterioration in optical quality) occurs during focusing. In the zoom lens systems of Patent Literature Nos. 3 and 4, since focusing is carried out using the third lens group, which has a relatively small diameter, the driving load for performing an autofocusing operation is small compared to that in the zoom lens systems in Patent Literature Nos. 1 and 2, which is advantageous; however, a large change in optical quality (deterioration in optical quality) still occurs during focusing.
On the other hand, a zoom lens system configured of five lens groups, i.e., a positive lens group, a negative lens group, a positive lens group, a negative lens group and a positive lens group, is disclosed in each of Patent Literature Nos. 3 and 5. However, since the balance of refractive power of the lens groups of these zoom lens systems is inappropriate, correction of various aberrations such as distortion and field curvature is insufficient.